linux-kernel - Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker

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Date:   Tue, 3 Jul 2018 21:39:02 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        stummala@...eaurora.org, gregkh@...uxfoundation.org,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Roman Gushchin <guro@...com>, mka@...omium.org,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Chris Wilson <chris@...is-wilson.co.uk>, longman@...hat.com,
        Minchan Kim <minchan@...nel.org>,
        Huang Ying <ying.huang@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>, jbacik@...com,
        Guenter Roeck <linux@...ck-us.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, lirongqing@...du.com,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker

On Tue, Jul 03, 2018 at 10:47:44AM -0700, Matthew Wilcox wrote:
> On Tue, Jul 03, 2018 at 08:46:28AM -0700, Shakeel Butt wrote:
> > On Tue, Jul 3, 2018 at 8:27 AM Matthew Wilcox <willy@...radead.org> wrote:
> > > This will actually reduce the size of each shrinker and be more
> > > cache-efficient when calling the shrinkers.  I think we can also get
> > > rid of the shrinker_rwsem eventually, but let's leave it for now.
> > 
> > Can you explain how you envision shrinker_rwsem can be removed? I am
> > very much interested in doing that.
> 
> Sure.  Right now we have 3 uses of shrinker_rwsem -- two for adding and
> removing shrinkers to the list and one for walking the list.  If we switch
> to an IDR then we can use a spinlock for adding/removing shrinkers and
> the RCU read lock for looking up an entry in the IDR each iteration of
> the loop.
> 
> We'd need to stop the shrinker from disappearing underneath us while we
> drop the RCU lock, so we'd need a refcount in the shrinker, and to free
> the shrinkers using RCU.  We do similar things for other data structures,
> so this is all pretty well understood.

<censored>
struct super_block {
...
        struct shrinker s_shrink;       /* per-sb shrinker handle */
...
}
<censored>

What was that about refcount in the shrinker and taking over the lifetime
rules of the objects it might be embedded into, again?